Not when the engine it is attached to has its highest performance at a lower peak than the maximum safe rpm, obviously.
This sentence makes no logic. Maybe you mean that you can achieve higher power output from stage 3 assuming that the engine won't explode or stay within the max allowed rpms? Then it is true, but this is not the subject of argument. There is enough 'free rpm range' for stage 3 to achieve more power and we assume that there is enough reliabilty as engines do not fail in GT6.
You understand how turbos improve engine performance, by providing more air for improved combustion, but you also understand that a point occurs where adding any more air no longer improves combustion, bearing in mind also that the function of the turbo is powered by the exhaust gasses?
The 'kit' consists of everything, needed for higher power output - larger turbo for higher air pressure and supposedly higher capacity fuel pump, etc, etc. This is the 'kit' and it must add more power. Or you want to say that there are tuning 'products' in the game, which cost a lot, but add no gain at all? Lets not get into that kind of logic...